The present invention relates to a sensor for measuring flow rate of fluid, and more particular, to a thermal air flow sensor.
As an air flow sensor provided in an intake manifold of an internal combustion engine in automobiles, etc. to measure an intake air quantity, a thermal air flow sensor has become the mainstream because of its ability of direct detection of a mass air quantity.
Recently, attention has been paid to air flow sensors manufactured by means of the semiconductor micromachining technology because of high speed responsibility and capability of detection of reverse flow making use of a high speed in responsibility.
However, it is known that thermal air flow sensors with the use of a semiconductor substrate are varied in flow output characteristics due to invasion of dirt substances, such as oil, dust, etc., and moisture onto a rear surface of a detecting element. JP-A-2003-270016 discloses a thermal air flow sensor, in which a hollow is provided in a plate-shaped substrate to form a diaphragm and a porous water proofing body is provided at an opening of the hollow.
With the conventional, thermal air flow sensor, when a detecting element comprising the plate-shaped substrate formed with the hollow and the diaphragm is bonded to a support, a clearance between the rear surface of the detecting element and a detecting element mounting surface of the support is liable to vary widely in dimension and it is difficult to control the clearance in dimension. When the clearance varies widely in dimension, air flowing into the hollow varies in quantity and direction, so that a curve is produced in a flow characteristic curve to cause a decrease in product yield.
In order to avoid the problem, it suffices to fully shield the hollow (diaphragm rear surface) from a passage chamber (a detection chamber in which a detecting part of the detecting element is arranged), through which air being a measuring object flows. That is, it suffices to put the hollow in a sealed state. Specifically, it suffices to bond whole periphery of the hollow by means of an adhesive in a manner to surround the hollow, thereby sealing the inside of the hollow. Since in general, detecting elements are rectangular-shaped, it suffices to bond all sides of a rectangular configuration.
When the whole periphery of the hollow is bonded by means of an adhesive to seal the inside of the hollow, however, there is caused a problem that complete sealing cannot be performed because variation in temperature and pressure causes air in the hollow (diaphragm rear surface) to expand in volume to result in breakage of the detecting element.
It is an object of the invention to provide a thermal air flow sensor, which is hard to be influenced by air flow being a measuring object and able to prevent breakage of a diaphragm due to variation in temperature and pressure.